Temperature-dependent dual-mode thermal management device with net zero energy for year-round energy saving

Reducing needs for heating and cooling from fossil energy is one of the biggest challenges, which demand accounts for almost half of global energy consumption, consequently resulting in complicated climatic and environmental issues. Herein, we demonstrate a high-performance, intelligently auto-switched and zero-energy dual-mode radiative thermal management device. By perceiving temperature to spontaneously modulate electromagnetic characteristics itself, the device achieves ~859.8 W m−2 of average heating power (∼91% of solar-thermal conversion efficiency) in cold and ~126.0 W m−2 of average cooling power in hot, without any external energy consumption during the whole process. Such a scalable, cost-effective device could realize two-way temperature control around comfortable temperature zone of human living. A practical demonstration shows that the temperature fluctuation is reduced by ~21 K, compared with copper plate. Numerical prediction indicates that this real zero-energy dual-mode thermal management device has a huge potential for year-round energy saving around the world and provides a feasible solution to realize the goal of Net Zero Carbon 2050.


Supplementary Note 1. Numerical modelling analysis of thermal management performance
There is a thermal balance relationship between thermal management device and the ambient as shown in Fig. 1a Here, Psun is absorbed solar radiation of the device, Patm(Tamb) is absorbed thermal radiation of the device from the atmosphere, Pdevice(Tdevice) is thermal radiation of the device, and Pparasitic(Tdevice,Tamb) is the parasitic heat, namely heat exchange between the device and the ambient by heat conduction and heat convection. Spontaneous thermal radiation of the device is determined by the temperature (Tdevice) and the emissivity of the device (εdevice(λ,θ)) simultaneously.
Adevice is the area of the device and   Supplementary Fig. 21). The radius of curvature is almost the same, when RC tape-2W SMP films are within the limited length (from 10 mm to 40 mm in this experiment).
The coiled RC tape-2W SMP films with the same radius of curvature will shade the same area of nano-Cr black Al plate. In other words, the device with the larger size has the higher solar heating power close to the prediction. As one might expect, the temperature rise of the device gradually decreases with the shrinking of active area in the device. The size of the device in next experiments is fixed at 40 mm × 40 mm to achieve an efficient thermal management performance.

Supplementary Note 3. Simulated test of dual-mode device without solar radiation indoors
To reveal the change of infrared emission power of dual-mode device in heating and cooling modes, an indoor experimental setup is designed to monitor temperature and estimate infrared emission power with the assistance of a feed-back controller ( Supplementary Fig. 12). The whole setup is sealed by a polyethylene (PE) film in a thermal insulating foam container to minimize the impact from thermal conduction and convection. Two same setups are used in parallel, where one is coated by a dual-mode device and the other is a same-sized aluminum (Al) foil with an infrared emissivity close to zero ( Supplementary Fig. 13).
For measurement of infrared emission power, a constant current source is supplied to the Al foil increasing from 0.1 A to 0.24 A with a step of 0.02 A, and the feed-back controller applies an offset Joule heating power on the dual-mode device to minimize the temperature difference between the dual-mode device and the Al foil ( Supplementary Fig. 15). The excess of Joule heating power (negative heat flux) is defined as the cooling power of dual-mode device from infrared radiation. As shown in Supplementary Fig. 16a, there is a cooling power of dual-mode device lower than 20 W m -2 at lower temperature, which enhances slightly with the temperature increase, when the temperature is lower than 51 ℃. Once the temperature is larger than 51 ℃, the cooling power rises steeply accompanying the unfold of RC tape-2W SMP film, and it continues to increase with the further increase of the temperature. The inflection temperature of 51 ℃ corresponds to the transformation of dual-mode device from heating mode to cooling mode. According to the infrared spectral characteristics of dual-mode device, the theoretical cooling powers of dual-mode device in two different modes is predicted (red line is heating mode and blue line is cooling mode). The experimental data agrees well with theoretical prediction in heating mode. It benefits from the negligible contact area between coiled RC tape-2W SMP film and Nano-Cr black coated Al plate, which maximums the effective area of dual-mode device for thermal management as much as possible. The lack of cooling power in cooling mode is primarily due to the insufficient thermal contact between unfolded RC tape-2W SMP film and Nano-Cr black coated Al plate, which decreases the cooling power of dualmode device in cooling mode in certain. We also measured the cooling power of dualmode device at different temperatures during the cooling process ( Supplementary Fig.   16b). There is a hysteresis of switching temperature inflection for cooling power between the heating and cooling processes. It is consistent with the hysteresis phenomenon of reversible shape memory of 2W SMP with the temperature (Supplementary Fig. 10).
Meanwhile, we investigated the influence on temperature, when dual-mode device switches from heating mode to cooling mode in this situation ( Supplementary Fig. 17).
The ambient temperature is ~29 ℃ and the calculated parasitic heat transfer coefficient (hc) is 17.5 W m -2 K -1 ( Supplementary Fig. 17a). The temperature of dual-mode device in heating mode is almost the same as that of Al foil. It validates that dual-mode device in heating mode has a very low heat radiative property close to ideal for solar heating.
When the temperature is over 50 ℃, the dual-mode device gradually switches to cooling mode resulting in the temperature difference between Al foil and dual-mode device. And this difference becomes larger and larger with the increase of Joule heating power (PJoule heating). The temperature difference could disappear again as the dual-mode device gets back into heating mode with the temperature decrease. The whole evolution process is consistent with that of cooling power, including the hysteresis phenomenon.
However, these facts show that the ability to radiate heat of dual-mode device exhibits a significant conversion, during the device switches back and forth between two thermal management modes.

Supplementary Note 4. Radiative heat loss and solar absorption
To have an obvious understanding of the impact of outer space and the sun in the nature on solar heating and radiative cooling performance of dual-mode device, we performed an analysis in theory based on the thermal balance relationship (Supplementary Equation (1)). This impact results in a certain difference between the simulated test indoors and the field test outdoors, although a simulated scene can be built indoors with the help of a solar simulator.
In the indoor environment, the device is surrounded by a roof, walls and some other objects, which have a combined infrared emissivity close to 100% in general.
Considering this situation, therefore, the overall indoor environment can be considered as a room-temperature (Tamb) thermal radiative source. This simplifies Supplementary In the outdoor environment, the existence of outer space at low temperature makes dual-mode devices have radiative heat loss by reducing the input of infrared radiation from the ambient to the device. Radiative heat absorption of the device from the ambient is only from the atmosphere, while the radiative radiation from out space is negligible due to its extremely cold temperature. In this scene, Supplementary Equation (3) is reducible to to the importance of reducing the infrared emissivity for solar heating. Meanwhile, an effective way arises for radiative cooling. On the other hand, solar radiation is a considerable energy source able to cause localized heating ( Supplementary Fig. 20b).
According to the solar radiation with ASTM G173 global solar spectrum, the huge potential on solar-thermal conversion is shown for dual-mode device in heating mode with a high solar absorption. On the contrary, high solar reflection is an essential spectral characteristic of dual-mode device in cooling mode for realizing radiative cooling under solar radiation.

Supplementary Note 5. Effect of incident angle of solar radiation on solar-thermal conversion
In this design, the switch of thermal management modes is realized by morphological evolution of RC tape-2W SMP film. For heating mode, the film rolls to one side to maximize the exposed area of the nano-Cr black Al plate. And for cooling mode, the film spreads out until completely covering the bottom nano-Cr black Al plate.
The device in cooling mode is almost isotropic in two-dimensional working plane. By  Supplementary Fig. 14).
The average temperature of Al foil is used as reference to calculate temperature change (ΔT=Tsample-TAl foil). Al foil, gray bar; Cu plate, orange bar; wood chip, green bar; cotton, purple bar; dual-mode device, color-graduated bar (heating mode is red boundary and cooling mode is blue boundary). The error bars represent the standard deviation of data in the last 1200 s within each period.
Supplementary Table 1